A tapered roller bearing in its simplest form has a cup (outer race) that is typically fitted to a housing, a cone (inner race) that is typically fitted over a shaft, and tapered rollers (rolling elements) organized in a single row between the cup and cone. The cup and cone have tapered raceways that the rollers along their tapered side faces contact. In addition, the cone at the large end of its raceway has a thrust rib against which the large end faces of the rollers bear, and it prevents the rollers from being expelled from the annular space between the raceways. The thrust rib forms an integral part of the cone and cannot be displaced with respect to the cone.
A single row tapered roller bearing has the capacity to support or transfer radial loads and also axial loads in one axial direction. A bearing system composed of two single row tapered roller bearings mounted in opposition will transfer not only radial loads, but also axial loads in both axial directions, and will thus confine that which rotates both radially and axially. When mounted in opposition, the bearings may assume either the indirect configuration or the direct configuration. In the indirect configuration the small ends of the rollers of the one bearing are presented toward the small ends of the rollers for the other bearing. In the direct configuration the two bearings have the opposite orientation. Irrespective of the mounting configuration, the bearings may be adjusted to a desired setting by shifting only one of the races of either bearing axially. The setting may be various degrees of end play, in which clearances exist within the bearings, or various degrees of preload, which are characterized by the absence of clearances and increased dynamic stiffness for the axis of rotation. Another condition of line-to-line contact, often referred to as zero end play, is difficult to maintain. A light preload is preferred, but too much preload can damage the bearings.
Assuming that the two bearings are mounted in the indirect configuration and that the shaft and cones rise in temperature above the temperature of the housing and the cups, the differential thermal expansion between the shaft and cones, on one hand, and the cup and housing, on the other, produces two counteracting disruptions to the setting for the bearings. First, the axial expansion of the shaft tends to reduce or eliminate the slight preload. On the other hand, the radial expansion of the cones tends to increase the preload. The amount that one prevails over the other depends on the axial spread between the bearings, the diameter of the cones, and the angles of the raceways. Generally speaking, the radial expansion more than offsets the axial expansion, and when it does the bearings acquire a greater preload.
With directly mounted bearings, both the axial expansion and the radial expansion contribute to an increase in preload.
Stated somewhat differently, tapered roller bearings generate internal axial reaction forces due to the raceway angles. This typically means that a pair of single row tapered roller bearings are used in opposition so that the axial forces may be cancelled out against each other. Also, this allows the pair of bearings to carry external loads in any direction of application. At setup, a careful and precise axial location adjustment of the opposed races relative to each other is required. This process is called bearing setting and produces either endplay, line-to-line contact, or preload.
While a slight preload is desirable for a pair of single row tapered roller bearings mounted on opposition, too much preload is detrimental. Differential thermal expansion between the cones and cups and the shaft and housing to which they are respectively fitted can produce excessive preload. That preload may:                reduce bearing life        increase bearing noise and vibration        increase bearing generated heat        increase bearing torque        increase damage to bearing lubricant        increase roller contact stress        increase cage damage        
Heretofore, efforts have been made to compensate for differential thermal expansion in bearing systems having opposed single row bearings. One involves installing a compensating ring having a high coefficient of thermal expansion behind one of the four races of the system. The thermal expansion of the compensating ring generally offsets the differential thermal expansion otherwise experienced by the bearing system, so that the system remains within acceptable tolerances. See U.S. Pat. No. 5,028,152. Another effort involves controlling the position of the thrust rib in one of the bearings with pressurized hydraulic fluid. See U.S. Pat. No. 3,716,280.